Method for producing gallium, particularly for semiconductor purposes



i for further use.

United States Patent 9 Ciairns. a. ass-a My invention relates to a method for producing gallium, preferably in the degree of purity desired for semiconductor purposes such as the use of gallium as a component of a semiconductor compound, for example GaSb, GaAs or GaP, or the use of gallium as a dopant for example in silicon or germanium.

Gallium has been recently gaining considerably in interest for semiconductor purposes. Although gallium is not one of the rarest elements, it is still looked upon as a rare element, because it can be gained only by rather complicated and expensive methods because of its widely dispersed occurrence in small concentrations. Ores, largely of gallium are not known. Aside from forming an 0.1 to 0.7% constituent of germanite, gallium occurs in nature as a regular concomitant to indium, germanium, iron, copper, manganese, tin, antimony and, above all, to zinc and aluminum. Almost all bauxites and zinc blendes contain between 20 and 500 p.p.m. gallium. For this reason, the sodium aluminate liquors of the Bayer aluminum oxide method are relatively rich in gallium and thus, in view of the large occurring quantitles, have become a much desired starting material for gallium production. By aluminate liquor is understood the liquor produced by dissolving bauxite in caustic soda. The following production methods have been proposed:

(1) Enrichment of gallate in the sodium aluminate liquor by precipitating the alumina, followed by tractionated precipitation of the residual liquor with soda. The major portion of the gallium then separates in the last fractions as Ga(OI-I) or Ga o -hydrate.

(2)' Electrolytic production of gallium from the gallium-rich aluminate liquors according to the method of Breteque. The gallium is precipitated on a mercury cathode of 12 qm. size while exactly maintaining accurately-determined current voltage and agitation conditions. An amalgam containing about 1% gallium is withdrawn, and gallium is extracted therefrom by means of hot sodium hydroxide. Gallium is then produced from the sodium gallate solution by electrolysis. From 70 ppm. gallium contained on the average per ton in the aluminate liquor, a quantity of about 60 ppm. can thus be obtained, the residual quantity remaining in the liquor.

(3) The sodium gallate of the aluminate liquor, together with other reducible impurities, is reduced to metal by means of sodium amalgam. The gallium dissolved in mercury and thus forming a constituent of the amalgam, is separated from the aluminate liquor. From the amalgam, the gallium is extracted with acidic or alkaline agents and precipitated electrolytically in metallic form.

For large-scale industrial production the above-mentioned methods exhibit various disadvantages:

The first-mentioned method cannot be performed within the normal course of the Bayer aluminum oxide process. The addition of soda spoils the residual liquor In addition, the fractional precipitation is time-consuming and expensive.

The other two methods, although applicable in the course of the Bayer aluminum oxide process, are incon- 3,170,857 Patented Feb. 23, 1965 venient and expensive because of the necessity of employing considerable quantities of mercury. In both of these latter methods, rather large mercury quantities must be kept in circulation to isolate relatively very small quantities of allium or to react such small galliumquantities with sodium. Particularly, the long duration of the electrolysis, the difiicult precipitation and agitation conditions and the inevitable coprecipitation of silicon in the second method, as well as the additional production of sodium amalgam and the circulatory pumping of large quantities of caustic soda and sodiumand-gallium-containing mercury in the last-mentioned method are particularly disadvantageous.

It is an object of my invention to provide a method for producing gallium from the sodium aluminate liquors of the Bayer aluminum oxide process which avoids the above-mentioned disadvantages of the known methods and which is also easily performed with a high degree of efiiciency. Another object of the invention, in conjunction with those mentioned, is'to obtain the resulting gallium in an extremely high degree of purity which makes the product particularly well suitable for purposes of semiconductor technology.-

According to a feature of my invention, the gallium contained in the form of sodium gallate in the sodium aluminate liquor of the Bayer aluminum oxide process is cemented and sorbed by an addition of active, finely distributed aluminum. Y The degree of fine distribution may be carried to the extent that aluminum dissolved in gallium is used. The aluminum addition is at least sufficiently large that the gallium, as it is being liberated, is completely bonded by aluminum in an aluminum-gallium alloy. From this alloy, the gallium is chemically or electrochemically separated.

The method according to the invention will presently be described in further detail.

The cementation of gallium from the aluminate liquors succeeds because of the more electro-positive character of aluminum as compared to gallium, namely according to the reaction equation A comparison of the normal potentials at Al/Al +-1.67 v. and Ga/Ga 0.52 v. shows that a potential difference of 1.15 v. is available for this reaction, disregarding the basic environment. This is approximately the same amount, as is attained relative to Na/Na when using sodium amalgam in accordance with the above-mentioned method. In this case, however, the use of mercury as an intermediate medium can be done away with.

The addition of active, finely distributed aluminum affords a rapid throughput of large quantities of alumihate-liquor.

The intermixing is further promoted by the gas evolution resulting from the side reaction whereas the aluminate liquor is not altered by the reaction product Na[Al(OH) which is of a similar type as the liquor (lye) itself. A redissolution of the precipitated gallium by the available free hydroxide ion according to the reaction is prevented by employing aluminum in excess to the gallate. The dimensioning therefore is so chosen that the resulting gallium is completely bonded by excess aluminum and hence is present as an aluminum-gallium alloy in'the reaction mixture. This alloy is separated from the aluminate liquor. The separation can be effected in known manner by filtration or centrifugation. If the aluminum-gallium alloy is still too rich in aluminum for the intended further fabrication, then the aluminum content can be reduced by treatment with alkali liquor or with diluted hydrochloric acid. This virtually dissolves only the aluminum because gallium dissolves in alkali liquor a multiple times more slowly than aluminum. If sodium hydroxide is used, the resulting aluminate liquor (lye) can directly be added to the liquor of the Bayer aluminum oxide process.

The methods described in the following are particularly well suitable for obtaining the gallium from the aluminum-gallium alloy.

The aluminum-gallium alloy is converted in a flow of chlorine or of chlorine-containing gas, into a mixture of aluminum (ED-chloride and gallium (ED-chloride. The gallium (ED-chloride is extracted with an aliphatic or hydroaromatic hydrocarbon, and gallium is obtained therefrom. More in detail, this method can be performed as follows. The aluminum-gallium alloy, separated from the liquor and in most cases of doughy consistency, having preferably an aluminum content of less than is placed into a quartz boat and is dried in a quartz furnace at 150 C. within a current of nitrogen. Thereafter, a hydrogen chloride or chloride current of moderate speed is passed over the alloy, and the furnace temperature is increased beyond 300 C. to such an extent that a continuous sublimation of the occurring gallium trichlorides and aluminum trichlorides takes place. These are caught and rinsed by means of nitrogen to eliminate adhering hydrogen chloride or chlorine. Thereafter, the trichlorides are dissolved in hexane. Also applicable for this purpose are, for example, pentane, heptane, cyclohexane or methylcyclohexane. The GaCl becomes rapidly dissolved, Whereas the AlCl virtually remains undissolved and forms a bottom body. By means of 1 liter of nhexane, more than 1 kg. of GaCl can be dissolved, for example. In contrast, only 22 milligrams of AlCl could be ascertained at 25 C. The solubility of any occurring other trichlorides, for example Fe-Cl and lnCl is still lower.

The GaCl /hexane solution is decanted off the AlCl and the latter is again washed with fresh hexane. The remaining AlCl can be further used, for example as Friedel-Crafts catalyst.

The GaCl /hexane solution can be converted to an aqueous alkali gallate solution from which the gallium can be obtained by electrolysis. If particularly pure gallium is to be produced, as required for example in semiconductor technology, the solution can be converted with aluminum-tri-alkyls to gallium tri-alliyls, and from these a highly pure gallium can be obtained by thermal dissociation as is further described in coassigned copending application Serial No. 108,996 of Dotzer et al.

A method of obtaining gallium particularly suitable for semiconductor purposes involves the electrochemical processing of the aluminum-gallium alloy in such a manner that the aluminum-gallium alloy is inserted as anode in an electrolysis cell with an aluminum-organic complexsalt electrolyte, and the gallium is obtained by the electroylsis as an anode residue in metallic form. Due to its more electro-positive character, the aluminum is preferentially dissolved from the liquid, mechanically stirred anode alloy, whereas the gallium remains. The method can be performed by placing the dried and melted Al-Ga alloy into an anode trough arranged above the aluminum sheet cathode within an electrolysis vessel. The supply of anode current is effected through a platinum wire passing through the hollow shaft of a stirrer structure of glass which takes care of continuous mixing and surface motion of the anode alloy. The electrolyte is filled into the vessel under a protective gas atmosphere of ntirogen or argon in the absence of humidity. The electrolyte consists, for example, of NaF.2Al(C H 01' (CH NCl.2Al(C H The electrolysis is performed at to C. with a cathode current density of approximately 1 A./dm. As soon as all of the aluminum is dissolved by the anodically evolving ethyl radicals and thus forms aluminum triethyl, the cell voltage increases in accordance with a new potential that adjusts itself for the dissolution of gallium. This increase in voltage indicates the termination of the aluminum dissolution. Only gallium is then contained in the anode trough. This remaining gallium can be further purified by electro-refining, for example in an (CH NF.2Ga(C H -electrolyte is described in copending coassigned application Serial No. 130,191 of Dotzer et a1.

According to another way of performing the process of the invention, the aluminum-gallium alloy can be liberated of the aluminum content by treatment with alkali liquor or diluted acid and the gallium be obtained in liquid form. Due to the fact that gallium is by 1.15 v. more noble than aluminum, the aluminum is almost exclusively dissolved out of the usually doughy-to-liquid gallium-rich alloy during stirring or shaking, so that relatively pure, liquid gallium remains.

The aluminum addition may be prepared from mechanically comminuted commercial aluminum or refined aluminum. Suitable is powdered aluminum having a particle size of several millimeters or alluminum solved in gallium. The aluminum should exhibit fresh, oxide-free surfaces and should, therefore, be highly active. When gallium contents below 1% by weight in the sodium aluminate liquor is used as the starting material, it is advisable to employ aluminum particles with less than 1 mm. diameter in order to have the greatest possible active aluminum surface available for the desired commentation. Particularly good results have been obtained with an aluminum addition which was finely ground together with aluminum alkyls under exclusion of air and humidity and which was stirred into the aluminate liquor with the adhering aluminum alkyls.

Particularly advantageous as aluminum addition is the aluminum powder resulting from the aluminum-alkyl pressure synthesis, this powder being permeated with aluminum alkyl. A product of this kind is particularly fine and highly active, although at the present it is usually being burned as a waste product of no further use. In industry, the. aluminum-alkyl pressure synthesis is employed on a large scale mainly for the production of aluminum-diisobutyl-hydride. The resulting aluminum powder, mixed with the strongly reducingly acting aluminum-diisobutyl-hydride, can be directly stirred into the sodium aluminate liquor of .the Bayer aluminum oxide process. The large aluminum surface, due to the high degree of distribution, results in a particularly rapid reduction of the gallate contained in the aluminate liquor and thereby considerably shortens the time needed for the method.

Another advantage of this type of aluminum addition resides in the fact that, aside from the gallium of the aluminate liquor, the gallium of the aluminum residue from the aluminum-alkyl pressure synthesis is also utilized, the Ga content of the latter being up to 1%.

The following example will further illustrate the cementation method for obtaining gallium according to the invention.

A 7.2 normal NaOH solution, containing 2.36% by weight of gallium dissolved in form of sodium gallate (1.99 g.) was given an addition of 1.5 times the quantity in weight of aluminum pieces (3.0 g.). The mixture at normal room temperature (20 C.) was stirred. After 10 hours the resulting compact Al-Ga alloy was separated from the liquor and briefly washed with water and analyzed. The alloy Was found to contain 10.4% Al and 79.6% Ga (1.95 g.). This corresponded to a 98.0% cementation of the gallium quantity originally available.

The cementation is preferably effected at normal room temperature in order to minimize the dissolution of aluminum With formation of hydrogen, which increases relatively rapidly with increasing temperature. If an increased consumption of aluminum per cemented gallium quantity is acceptable, then the cementing can also be performed at temperatures up to 80 C.

1 claim:

1. A method for producing gallium from the sodiumaluminate liquor of the Bayer aluminum oxide process for recovering alumina from aluminum ores, which comprises adding finely divided aluminum to the sodium aluminate liquor to cement and to absorb the gallium contained in said sodium aluminate liquor as sodium gallate, the quantity of aluminum added being at least large enough to completely absorb the evolving gallium as an aluminum-gallium alloy, and thereafter separating the gallium from said aluminum-gallium alloy.

2. The method of claim 1 wherein the gallium is separated from the aluminum-gallium alloy by adding a reagent to dissolve the aluminum content thereby obtaining liquid gallium.

3. The method of claim 1 wherein the aluminum to be added is finely ground under exclusion of air and moisture with aluminum-alkyls, and th aluminum powder together with the adhering aluminum-alkyls is stirred into the alnminate liquor.

4. Method of claim 1 wherein the aluminum addition results from aluminum-alkyl pressure synthesis and is permeated with aluminum-alkyl.

5. A method for producing gallium from the sodiumaluminate liquor obtained in the Bayer aluminum oxide process of recovering alumina from aluminum ores which comprises adding finely divided aluminum to the sodium aluminate liquor to cement and absorb the galllium contained in said sodium aluminate liquor as sodium gallate, the quantity of aluminum added being at least large enough to completely absorb the evolving gallium as an aluminum-gallium alloy, subjecting the aluminumgallium alloy to a flow of chlorine-containing gas, thereby converting the alloy to a mixture of aluminum (III)- chloride and gallium (ED-chloride, extracting the gallium HID-chloride by a hydrocarbon solvent selected from the group consisting of hexane, pentane, heptane, cyclohexane and methylcyclohexane, and separating the gallium from the solution.

6. A method for producing gallium from the sodiumaluminate liquor obtained in the Bayer aluminum oxide process of recovering alumina from aluminum ores which comprises adding finely divided aluminum to the sodium aluminate liquor to cement and absorb th gallium contained in said sodium aluminate liquor as sodium gallate, the quantity of aluminum added being at least large enough to completely absorb the evolving gallium as an aluminum-gallium alloy, subjecting the aluminumgallium alloy to a flow of chlorine-containing gas, thereby converting the alloy to a mixture of aluminum (III)- chloride and gallium (ED-chloride, extracting the gallium (llD-chloride by a hydrocarbon solvent selected from the group consisting of hexane, pentane, heptane, cyclohexane and methylcyclohexane, adding alkali liquor to convert the gallium (III)-chloride/hydrocarbon solution into an aqueous alkali gallate solution, and separating gallium from gallate solution by electrolysis.

7. A method for producing gallium from the sodiumaluminate liquor obtained in the Bayer aluminum oxide process of recovering alumina from aluminum ores Which comprises adding finely divided aluminum to the sodium aluminate liquor to cement and absorb th gallium contained in said sodium aluminate liquor as sodium gallate, the quantity of aluminum added being at least large enough to completely absorb the evolving gallium as an aluminum-gallium alloy, subjecting the aluminumgallium alloy to a flow of chlorine-containing gas, thereby converting the alloy to a mixture of aluminum (ill)- chloride and gallium (HQ-chloride, extracting the gallium (MD-chloride by a hydrocarbon solvent, adding aluminum trialkyl to the gallium (Ill) chloride/hydrocarbon to conv rt the gallium (III) chloride into gallium trialkyl and thermally dissociating said. gallium triallzyl to yield gallium.

8. A method for producing gallium from the sodiumaluminate liquor obtained in the Bayer aluminum oxide process of recovering alumina from aluminum ores which comprises adding finely divided aluminum to the sodium aluminate liquor to cement and absorb th gallium contained in said sodium aluminate liquor as sodium gallate, the quantity of aluminum added being at least large enough to completely absorb the evolving gallium as an aluminum-gallium alloy, using the aluminum-gallium alloy as the anode in an electrolysis cell containing an aluminum-organic complex-salt electrolyte selected from the group consisting of NaF.2Al(C H and (CH NCl.2Al(C l-l and separating gallium as the electrolysis anode residue.

9. A method for producing gallium from the sodiumaluminate liquor obtained in the Bayer aluminum oxide process of recovering alumina from aluminum ores which comprises adding aluminum dissolved in gallium to the sodium aluminate liquor to cement and absorb the gallium contained in said sodium aluminate liquor as sodium gallate, the quantity of aluminum added being at least large enough to completely absorb the evolving gallium as an aluminum-gallium alloy, and thereafter separating the gallium from said aluminum-gallium alloy.

Reierences Cited by the Examiner UNITED STATES PATENTS 2,793,179 5/57 Breteque 204- 2,798,845 7/57 Breteque 204- 2,898,278 8/59 Plust 75-84 2,944,948 7/60 Giraitis 204-59 2,952,589 9/60 Ziegler 204-59 2,985,568 5/61 Ziegler 204-59 3,069,334 12/62 iegler 204-59 FQREIGN PATENTS 1,111,402 7/ 61 Germany.

OTHER REFERENCES Handbook of Chemistry and Physics, 27th Edition, 1943, Chemical Rubber Publishing Company, Cleveland, Ohio, pages 13681370.

Hoffman: US. Bureau of Standards Research Papers No. RP 734, vol. 13, No. 5, pages 665-672, November 1934.

Lysenko et al.: Separation of Gallium from Other Elements, translated from Zavockkaya Laboratoriya, vol. 26, No. 2, pages -147, translation found in Industrial Laboratory, vol. 26, No. 2, pages 151-154, December 1960.

Powell et al.: The Extraction and Refining of Germanium and Gallium, J. of Applied Chemistry, December 1, 1951, pages 541-551.

BENJAMIN HENKIN, Primary Examiner. 

1. A METHOD FOR PRODUCING GALLIUM FROM THE SODIUMALUMINATE LIQUOR OF THE BAYER ALUMINUM OXIDE PROCESS FOR RECOVERING ALUMINA FROM ALUMINUM ORES, WHICH COMPRISES ADDING FINELY DIVIDED ALUMINUM TO THE SODIUM ALUMINATE LIQUOR TO CEMENT AND TO ABSORB THE GALLIUM CONTAINED IN SODIUM ALUMINATE LIQUOR AS SODIUM GALLATE, THE QUANTITY OF ALUMINUM ADDED BEING AT LEAST LARGE ENOUGH TO COMPLETELY ABSORB THE EVOLVING GALLIUM AS AN ALUMINUM-GALLIUM ALLOY, AND THEREAFTER SEPARATING THE GALLIUM FROM SAID ALUMINUM-GALLIUM ALLOY. 